De-embedding
Removing the test fixture's own response from a measurement so that only the device under test remains. In photonics this usually means subtracting the fiber-to-chip couplers, measured on a dedicated reference structure, from every on-chip device measurement.
No fiber-coupled measurement of a photonic chip measures the device alone. Light enters through a coupler, crosses the device, and leaves through another coupler, so the raw number is always
and the couplers are rarely small: a grating coupler costs several dB and, worse, has a strongly wavelength-dependent, roughly Gaussian passband that imprints itself on every spectrum measured through it. De-embedding is the removal of those fixture terms.
The reference structure. The standard fixture calibration is a back-to-back (loopback) structure on the same die: two nominally identical couplers connected by a short waveguide and nothing else. Its measured loss, minus the negligible waveguide term, is two couplers' worth, so the per-coupler insertion loss spectrum is half the loopback loss in dB. Subtracting two of these curves from a device path leaves the device. Because subtraction in dB is division in linear power, spectral de-embedding is a division of transmission spectra, which is why the reference must be measured over the same wavelength grid, same polarization, and ideally the same fiber landing.
The identicality assumption. The subtraction assumes the reference couplers match the device-path couplers. Within one die the agreement is typically a few tenths of a dB; across a wafer, thickness and linewidth gradients shift both the peak loss and the center wavelength of the passband, which is why wafer-level test flows place a loopback in every reticle rather than one per wafer. Alignment repeatability sets a separate floor: if repeated landings on the same coupler scatter by 0.1 dB, the de-embedded device numbers inherit at least that uncertainty. The scatter is worth measuring once, by landing several times, and quoting alongside the results.
Reading the ripple. Periodic structure on a de-embedded (or raw) spectrum usually indicates a parasitic Fabry-Perot cavity: two weak reflectors form a resonator whose fringe spacing obeys . The period, together with the group index, gives the cavity length , which generally identifies the reflecting pair; a length matching the coupler separation implicates coupler back-reflection, one matching the chip length implicates the facets. The peak-to-peak amplitude bounds the product of the two reflectivities.
Beyond couplers. The same pattern appears throughout photonic metrology: the cutback method removes coupling by comparing structures that differ only in length; RF characterization removes probes and pads with short, open, and thru structures; detector calibration removes the photodiode's spectral response. In each case the measurement includes its fixture, and a deliberately designed reference is what permits taking the fixture back out.
References: Chrostowski & Hochberg, Silicon Photonics Design (2015), Ch. 9; the companion procedure with worked numbers is De-embedding coupler loss with reference structures, and the Grating Coupler Spectrum Fitter extracts per-coupler loss, bandwidth, and ripple from a pasted loopback scan.